The present invention relates to an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus and to a developing agent used in the image forming apparatus, particularly, to an image forming apparatus in which the development is performed by a magnet rotation developing system and a two-component developing agent used in the image forming apparatus, said developing agent containing a magnetic toner and a carrier.
An image forming apparatus using a magnet rotation type developing system is disclosed in, for example, Japanese Patent Publication (Kokoku) No. 7-40156. The apparatus disclosed in this prior art comprises a developing roller arranged to face an image carrier carrying an electrostatic latent image. The developing roller includes a hollow cylindrical rotatable sleeve made of a non-magnetic material and a magnetic roll arranged within the sleeve, having a plurality of magnetic poles, and rotatable independently of the sleeve.
In the magnet rotation developing system, a two component system developing agent consisting of a magnetic carrier and a magnetic toner having a magnetic material added to the surface of a magnetic toner particle is applied to a developing roll, and the magnetic roll and the sleeve are rotated in the same or opposite directions so as to supply the developing agent into a developing region while allowing the developing agent particles to rotate about their own axes. In the magnet rotation type developing system of this type, the toner specific concentration can be controlled easily, compared with the ordinary magnet stationary developing system, making it possible to increase the toner specific concentration. It follows that the toner transfer amount is increased so as to improve the developing efficiency. Where the magnet roll and the sleeve are rotated in the same direction, the rotating direction of the developing agent about its own axis is opposite to the transfer direction of the developing agent. Where the magnet roll and the sleeve are rotated in the opposite direction, however, the rotating direction of the developing agent about its own axis is equal to the transfer direction of the developing agent, making it possible to further increase the transfer amount of the developing agent. Therefore, the particular rotating directions of the magnet roll and the sleeve is adapted for a high speed development.
In general, the toner specific concentration on the developing roll is about 6% by weight in the two component developing agent of the magnet stationary developing system. On the other hand, the toner specific concentration on the developing roll can be maintained at about 50% in the two component developing agent of the magnet rotation developing system, leading to the merit that a so-called xe2x80x9ccarrier attachmentxe2x80x9d, i.e., the phenomenon that the carrier is attached to the developing roll, is unlikely to take place.
It is of high importance in this technical field to form stably excellent images high in image concentration and low in fogging by using the two component system developing agent of the magnet rotation developing system.
An object of the present invention, which has been achieved in view of the situation described above, is to provide a developing agent capable of forming images high in image concentration and low in fogging.
Another object of the present invention is to provide an image forming apparatus that permits forming images high in image density and low in fogging.
According to a first aspect of the present invention, there is provided a developing agent, comprising a toner particle containing a magnetic powder and a binder resin, wherein the developing agent has a kinetic resistance of 200 Mxcexa9 or less and the absolute value of the difference xcex94V between the maximum value and the minimum value of the surface potential of the developing sleeve supporting the developing agent is not larger than 5V.
According to a second aspect of the present invention, there is provided an image forming apparatus, comprising:
at least one image carrier;
a developing device including a hollow cylindrical rotatable sleeve arranged to face the image carrier, housing a developing agent including a toner particle containing a magnetic powder and a binder resin, the developing agent having a kinetic resistance of 200 Mxcexa9 or less, and the absolute value of the difference xcex94V between the maximum value and the minimum value of the surface potential of the developing sleeve supporting the developing agent being 5V or less, and a magnet roll rotatable independently of the sleeve and having a plurality of magnetic poles, the developing agent supported on the developing sleeve being supplied to an electrostatic latent image formed on the image carrier so as to form a developing agent image;
a transfer device for transferring the developing agent image into a transfer material; and
a fixing device for fixing the transferred developing agent image.
The image forming apparatus of the present invention makes it possible to form developing agent images satisfactory in image density and low in fogging.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.